Particle acceleration having low electron gain



Jan. 6, 1970 G. w. sbmcH 3,488,509

PARTICLE ACCELERATION HAVING LOW ELECTRON GAIN Filed Dec. '7, 1964 3 Sheets-Sheet l DIRECT VOLTAGE SOURCE INVENTOR. GEORGE w. GOODRICH ATTORNEY fig.

Jan. e, 1970 G. w. @QDRICH` 3,488,509

PARTICLE ACCELERATION HAVING LOW ELECTRON GAIN Filed oec. 7, 1964 5 sheetssneet 2 LENGTH OF ACCELERATOR PARABOLA INVENTOR. LENGTH oF AccELERAToR -GEORGE W. GOODRICH ATTORNEY Jan. 6, 1970 G. `w. GooDRlcH 3,488,509

4P'AITVI'ICLE. ACCELERATION HAVING LOW ELECTRON GAIN Filed Dec. v, 1964 s sheets-sheet s 62 l Q 42 64 Q Q Fig. 7 ELr-:cTRoN BEAM 65 se 66 VOLTAGE souRcE DIRECT VOLTAGE SOURCE DIRECT VOLTAGE SOURCE Frgs4 INVENTOR. GEORGE W. GOODRICH BY ATTORNEY United States Patent O U.S. Cl. Z50- 213 11 `Claims ABSTRACT F THE DISCLOSURE Particle accelerator devices such as small diameter semiconductor tubes which can be mosaically bonded together to accelerate and maintain the spatial distribution of spatially distributed incident particles while having low gain.

In typical image intensier devices a photo-electron image, which may .be produced by focusing a radiant image upon a photocathode, is formed having a particular spatial density distribution corresponding to the intensity gradations of the radiant image. The electrons comprising this photo-electron image are subsequently multiplied or otherwise accelerated to higher energies prior to striking a phosphor screen and producing an intensified image. An ever present technological problem in these devices is the tendency of the photo-electron image to become blurred during its passage from photocathode to phosphor screen so that the intensified image does not precisely correspond to the radiant intensity variations of the incident image. This blurring of the electron image begins when a radiant energy beam strikes the photocathode releasing electrons having different energies and angles of emission and continues in transit through the device unless offset by conventional electrostatic and/or magnetic lenses. These lenses, however, are disadvantageous in that they have great size and bulk, and only limited image resolution can be obtained within the limitations of practical operating voltages and eld strengths.

The present invention is concerned with a device for accelerating spatially distributed electrons or other charged particles to higher energies without distorting their space orientation. That is, by placing the invention, for example, between the photocathode and phosphor screen of a proximity focused electrostatic image intensier, the photoelectron image will -be constrained to more nearly retain its original spatial orientation during its transit to the phosphor screen, as hereinafter described in greater detail, without requiring conventional focusing techniques.

When a curve is plotted of the electron gain Vs. the applied voltage across the electron path, the gain increases above unity, reaches a peak and then returns to unity. This invention teaches that if the energy of the accelerated electrons is in the range where the gain has returned to unity, then the accelerating voltage is sufficiently high to provide an accelerator. As mentioned, the gain first increases to a maximum and then returns to unity, remaining there until the applied voltage becomes so high there is a voltage breakdown. The accelerating field can be obtained by applying a voltage across a surface having a constant resistance gradient or -by applying a voltage across a surface having an increasing resistance gradient, with the voltage across the latter surface being substantially less, for reasons later explained.

In a preferred embodiment of this invention, a plurality of glass tubes in parallel relation, termed a mosaic, are coated on their inner surfaces with a resistive material having a resistance gradient, which is resistance change per unit length, which increases in a predetermined man- ICC ner along the longitudinal dimension of the tube. Upon application of "a suitable voltage between the ends of the mosaic, a radial electric field is generated within each tube which tends to direct a charged particle toward the central axis of the tube while the longitudinal electric eld accelerates the particle longitudinally down the tube. The trajectory of the particle will be oscillatory in the transverse plane, and for a certain range of input positions and energies the particle will not touch the tube wall. Thus, by providing a suiciently rapid increase in resistivity as a function of distance measured from the input plane of each tube it is possible to prevent particles which either enter the tubes externally or which originate from the tube Walls near the input section, from subsequently striking the walls before they exit from the tube with increased energy. The radial field generated in this embodiment minimizes the voltage needed to obtain acceleration of the charged particles without the loss 'of the particles to the -walls of the accelerator or loss bution.

It is therefore an object of the invention to provide a device for accelerating particles to higher energies.

An additional object of the invention is to provide a mosaic particle accelerator which accepts particles having a given spatial distribution and discharges them with substantially the same spatial distribution.

A further object of the invention is to provide a particle multiplier-accelerator device for multiplying and accelerating particles to higher energies.

Other objects and advantages will become apparent from the following detailed description and from the appended claims and drawings:

FIGURE l is a sectioned View of a preferred embodiment of my invention having a constant resistive gradient along the wall surface;

FIGURE 2 is a graph showing the voltage vs. gain and energy in an accelerator tube;

FIGURE 3 is a sectioned view af a single particle accelerator in accordance with the invention showing electric field lines and paths of an accelerated charged particle in an embodiment having an increasing resistance gradient;

FIGURE 4 is a graph showing the longitudinal potential gradient, dV/dL along the Walls of the accelerator as a function of distance from the accelerator input plane;

FIGURE 5 is a graph showing the radial potential gradient, dV/dr, as a function of ydistance from the accelerator input plane;

FIGURE 6 shows a continuously increasing potential distribution along the walls of an accelerator having` a specific potential gradient shown in FIGURE 4;

FIGURE 7 shows a mosaic of particle accelerator elements;

FIGURE 8 is a sectioned view of a single particle mulof their spatial distritiplier-accelerator device showing equipotential lines and particle trajectories; and

FIGURE 9 shows an image intensifierl tube comprised of a mosaic of particle multiplier-accelerator elements.

EMBODIMENT OF FIGURE l In the drawings in FIGURE l is shown a preferred embodiment of my invention having a particle source 20 which emits particles to be multiplied, such as electrons, into a tube 22 having an outer insulative wall 24 made of a material such as glass and an inner resistive coating 26, such as lead oxide or tin and antimony oxides. A collector 28 is positioned at the opposite end of tube 22 to receive the accelerated particles. A voltage source 30 is placed across tube 22 thereby causing the particles to be accelerated towards the anode 28. The anode 28 is made slightly higher in potential by ineans of battery 32 in order to attract the electrons thereto. The elements 20, 22 and 28 are hermetically enclosed in a reduced pressure region by tube means, not shown but similar to that shown in FIGURE 9 in a suitable manner.

In order that the embodiment shown in FIGURE l act as an accelerator the potential applied by battery 30 f should, for a particular tube length to tube diameter ratio, be suiciently high to cause the tube to return to unity gain. This can be better understood by looking at FIG- URE 2. wherein there is shown a graph having along its abscissa the voltage of battery 30 and having along the ordinate two sets of measurements, one being gain and the other being exit energy. Curve G shows the gain versus battery 30 voltage relationship and curve E shows the energy of an electron leaving the tube 22 versus battery 30 potential. When the voltage range of battery 30 is between zero and M, it is seen that the gain, or the ratio of exit electrons to entrance electrons, increases to a value substantially higher than unity. This is due to the electrons hitting the wall 26 and causing secondary emission resulting in a high gain or multiplication at ianode 28. However, as the voltage across the tube 26 is increased, the gain begins to decrease until at voltage N it is back to unity, meaning that the number of electrons entering is approximately equal to the number of electrons leaving tube 22, and in fact the electrons leaving the tube are the same electrons that entered the tube but at an accelerated rate. At voltage N it is also seen that the energy of electrons leaving the tube, curve E, has increased substantially. From voltage N and higher, the tube 22 performs an acceleration function resulting in the ability to increase the energy of particles while maintaining their resolution.

Instead of measuring the voltage of battery 30 along the abscissa, electron current in wall 26 could be measured to obtain lthe curves of FIGURE 2. Also, the ordinate could be defined as the energy of exciting electrons at the output end. Exciting electrons are those which hit the wall 26 and excite secondary electrons.

EMBODIMENT OF FIGURE 3 With the embodiments next described, the accelerating voltages may be decreased due to the fact a radial iield is formed in the accelerating tube which tends to prevent the electrons from hitting the tube Wall and therefore can continually increase their longitudinal velocity. Also, resolution is maintained since the electrons leaving the tube are traveling in a direction nearly parallel to the tube axis.

FIGURE 3 shows a single element particle accelerator 42 positioned between and in alignment with an electron source 44, such as a photocathode, and a collecting electrode 46 which may be a phosphor screen. The accelerator 42 is shown as a tube symmetric about the central axis 4S comprising an electrical insulative material 50 coated internally with a film of electrically resistive material 52, which may be of a predominantly tin, antimony, or lead oxide composition. Material 52 is shown decreasing in radial thickness in an axial direction from source 44 to electrode 46 providing an increasing resistivity. A direct voltage source 54 furnishes suitable biasing voltages such as volt, 100 volts, 5000 volts, and 5100 volts which are respectively applied to the electron source 44, the coating 52 at the input of accelerator 42, the coating 52 at the output of accelerator 42, and the collecting electrode 46. Current ows through the increasingly resistive material 52 due to the potential impressed across the accelerator resulting in an incremental potential drop per incremental unit length, that is, a potential gradient, which increases as a function of distance along the length of the accelerator. As a consequence of this gradient in a longitudinal direction along the walls of the accelerator, iield lines 56 emanate from material 52 decreasing in angle with the surface of material 52 and becoming more numerous with increasing distance from the input plane of the accelerator. Thus, as will be apparent to those skilled in the art an increasing longitudinal potential gradient, iV/dL where V is potential and L is axial displacement, directed along the walls of the accelerator toward the accelerator output plane and a radial potential gradient, dV/dr where r is radial displacement, directed toward the accelerator axis `48 is formed within the tube. The dependence of the respective potential gradients upon distance from the accelerator input plane is shown in FIGURES 4 and 5 hereinafter explained. Equipotential lines 58 which are everywhere perpendicular to the field lines 56 are also shown increasing in radii of curvature as a function of increasing field intensity. An electron entering the potential iield of the accelerator will therefore be accelerated toward the output plane of the accelerator by the increasing axial gradient dV/dL while at the same time the radial gradient dV/dr directs the electron toward the central axis 48. Its trajectory will be oscillatory in a transverse plane and Will be dependent upon the magnitude of the respective potential gradients as hereinafter discussed in greater detail with reference to FIGURES 4 and 5. However, for a certain range of input positions and energies the electron will not be able to touch the tube wall and its energy will be increased in proportion to the total potential impressed across the length of the accelerator. A typical general trajectory is shown by line 60.

Although resistive material 52 is shown placed upon the internal wall of material it will be appare-nt to those skilled in the art that the same effective electric iield will be created within the accelerator if the resistive material is placed upon the external wall of the tube.

DISCUSSION OF FIGURES 4-6 In the graph of FIGURE 4 the longitudinal potential gradient, dV/dL along the walls of the accelerator iS shown along the ordinate axis as a function of distance along the length of the accelerator from the input plane, I, to the output plane O. A corresponding graph of the radial potential gradient, dV/dr, is shown in FIGURE 5. In the figures, the corresponding longitudinal and radial potential gradients for an accelerator tube having a specified increasing resistivity pattern are designated by the same letters.

In FIGURE 4 line AL represents the condition wherein the longitudinal potential gradient, (lV/dL increases linearly with the distance along the accelerator from the input plane I to the output plane O. Such a potential iield is commonly called a square law iield since, as will be apparent from an integration of the pOtential gradient relationship, that is, where dV/a'L is in linear relation to L, the potential V varies along the length of the accelerator as the square of the distance from some original plane as shown in FIGURE 6. The radial potential gradient, dV/dr corresponding to this square law eld is shown by the line Ar in FIGURE 5 to be constant along the length of the accelerator. Since by definition the electric force E is directed opposite to the direction of potential gradient, the transverse trajectory of an electron through a square law iield will be analogous to the oscillation of a simple harmonic oscillator. That is, where the force acting on an electron is opposite and proportional to the displacement, or dV/dL=-E=KL, where K is a proportionality constant, the oscillation will amount to simple harmonic motion and the electron trajectory will be sinusoidal having a constant frequency and amplitude. The general electron trajectory 60 shown in FIGURE 3 however, should be understood to represent only a portion of the oscillation period.

Variations from the square law iield are shown in the figures by lines BL, Br, CL and Cr. Lines BL and C1, of FIGURE 4 respectively represent the conditions wherein the longitudinal potential gradient along the accelerator increases exponentially greater than and less than the square of the distance from some original plane. The corresponding radial potential gradients are shown in FIGURE 5 by lines Br and Cr.

For the potential gradient condition represented by line BL, that is, where dV/dL increases exponentially faster than the square of the displacement, the electron trajectory 60 in FIGURE 3 will be sinusoidal having an increasing frequency and a decreasing amplitude. Conversely, for the potential gradient condition represented by line CL the electron trajectorythrough the accelerator will be sinusoidal having a decreasing frequency and an increasing amplitude. As before, the electron trajectory 60 shown in FIGURE 3 should be understood to represent only a portion of the sinusoidal period.

The potential distribution existing along the wall of an accelerator having a longitudinal potential gradient illustrated by line AL of FIGURE 4 is shown in FIG- URE 6. That is, when the longitudinal'potential gradient dV/dL increases linearly with displacement along the accelerator wall, the potential V along the wall will continuously and smoothly increase in parabolic fashion as shown by curve A in the figure.

EMBODIMENT OF FIGURE 7 64 and output faces 65 are then coated with a thin metallic film, for example, by vapor deposition, so that all the elements are commonly connected both in the input and output planes and only two lead wires 66 and 68 are required to distribute the potential across all elements of the mosaic. Of course the metallic coating of each face 64 and 65 is in contact with the inner resistive surface of each accelerator.

EMBODIMENT OF FIGURE 8 `In FIGURE 8 a channel electron multiplier, as disclosed in copending application entitled Electron Multiplier, Patent No. 3,128,408, issued Apr. 7, 1964 to W. C. Wiley and now myself, is shown in combination with an accelerator element as hereinbefore discussed. The combination multiplier-accelerator element 70 is shown positioned between a cathode 72 and a phosphor screen 74 in electrical contact with the output plane of the element. Like the accelerator of FIGURE 3, element 70 is symmetrically tubular in shape and is comprised of an insulative material 76, such as (glass, Hcoated internally with an electrically resistive material 78 which possesses a uniform resistivity for a length M and a gradually increasing resistivity for a length A. A direct voltage source 80 supplies suitable biasing voltages such as 0 volt, 100 volts, and 7000 volts which are respectively applied to the cathode 72, the input of element 70, and the phosphor screen 74. Current flows through the resistive material 78 due to the potential impressed across the multiplier-accelerator resulting in a uniform incremental potential drop per incremental unit length in the M portion of element 70, changing to an increasing gradient in the A section in accordancewith the discussion related to FIGURE 3. The resulting electric field within the combination multiplier-accelerator element is illustrated in the figure by electric field lines. In the M region the lines are substantially parallel to the walls of element 70, where the electric field is of a uniform intensity, becoming curved in the A region where the electric field increases in intensity with displacement toward the phosphor screen 74. An electron entering the field from the cathode 72 will therefore not be restrained from striking the Walls in the M section becoming multiplied therein by means of secondary emission, as explained in the referenced patent supra; whereupon the multiplied electrons encounter the accelerating field within section A wherein they are constrained to avoid Contact with the walls and proceed to strike the phosphor screen 74 with increased energy. Lines 84 illustrate typical electron trajectories through the field. The electron impacting against the tube wall at 86 in section M is shown releasing two secondary electrons. One of these secondary electrons is shown striking the wall at 88, again releasing two secondary electrons which are focused by the field in section A; the other electron is shown missing the wall also being focused by the field in section A. The advantage of this embodiment is brought out more clearly in FIGURE 9 which shows a mosaic of many elements 70 in an image intensifier tube.

EMBODIMENT OF FIGURE 9 In FIGURE 9, a mosaic 90 of multiplier-accelerator elements 70 is positioned within the environment of a vacuum envelope 92 between a photocathode 94 and a phosphor screen 96. The constant potentials, 0 volt, 100 volts, and 5000 volts are supplied by a direct voltage source 98 respectively to the photocathode 94, the multiplication input of the mosaic 90, :and the phosphor screen 96 which is in electrical contact with the accelerator output plane. Thus electrons leaving the photocathode 94 spatially distributed in intensity proportional to the input image upon the cathode, enter the multiplication input plane 100 of the mosaic 90 wherein they are multiplied to a factor as great as 10,000,000 prior to entering the mosaic accelerating section where they are accelerated to a predetermined energy level prior to striking the phosphor screen. An output image is thereby produced on the screen 96 which is substantially brighter than the relatively weak input image and which retains the spatial intensity gradation of the input image without requiring conventional extrinsic focussing techniques.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other a-pplications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

Having thus described my invention, I claim:

1. Apparatus for accelerating charged particles to higher energies comprising a plurality of accelerators each having a first and a second opening for respectively receiving and discharging charged particles and an intervening particle channel,

said accelerators having their first openings disposed in a surface to form a particle beam input,

said accelerators having their second openings disposed in a plane to form a particle beam output,

means for providing a longitudinal potential gradient along at least a portion of the surface of said intervening particle channel which changes as a function of distance along said portion for providing a field within each accelerator that reduces particle impact with the accelerator walls,

and a collector disposed to receive the accelerated particles emanating from said particle beam output.

2. The apparatus of claim 1 with photocathode means for receiving a light image and emitting a corresponding electron image,

said photocathode means being placed at the first opening of said accelerators to emit the electron image into the first openings of said accelerators.

3. The apparatus of claim 2 with enclosure means for hermetically enclosing the accelerators.

4. The apparatus of claim 1 with said accelerators having adjacent their first openings a means for providing a constant potential gradient to provide a multiplying iield adjacent the changing longitudinal potential gradient.

5. Apparatus for accelerating particles to higher energies comprising a plurality of accelerators each comprising,

a hollow tube having input and output openings disposed to receive and discharge the accelerated particles,

said tube having a continuous resistive surface,

and means for providing a continuous current flow along the continuous resistive surface of said tube between the input and output openings,

the resistance of said resistive surface being varied so that the current ow therethrough establishes a longitudinal potential gradient along the entire surface of said tube which changes as a function of axial displacement,

and a collector disposed to receive the accelerated particles emanating from said output surface.

6. The apparatus of claim 5 with said accelerators having adjacent their iirst openings a means for providing a constant potential gradient to provide a multiplying field preceding the changing longitudinal potential gradient.

7. Apparatus comprising wall means of secondary electron emissive material defining an accelerating path having its longitudinal dimension substantially larger than its lateral dimension,

entrance means into the accelerating path defined by said wall means,

exit means from said accelerating path defined by said wall means spaced from said entrance means by said longitudinal dimension,

said accelerating path being totally clear of a lield producing wire on the longitudinal axis of the accelerating path,

said accelerating path being free of any superimposed oscillating electric field in a direction transverse to the longitudinal axis,

means for accelerating electrons along said accelerating path and for substantially increasing the energy of the electrons comprising means for producing a longitudinal electrical current liow in said Wall means to establish in the accelerating path a total field which nis an electric field having a component parallel to the Wall means in its longitudinal direction,

said longitudinal electrical eld component being sutiiciently large so that said energy of the electrons is increased substantially and so that the electron gain of said apparatus is substantially equal to unity.

8. Apparatus comprising a plurality of channels each having a first and a second opening for respectively receiving and discharging particles and an intervening particle channel,

said channels having their Ifirst openings disposed in a surface to form a particle beam input and output,

said channels having their second openings disposed in a plane to form a particle beam output,

means for providing a changing longitudinal potential gradient along the surface of said intervening particle channel,

and a collector disposed to receive the particles emanating from said particle beam output.

9. Apparatus comprising wall means of secondary electron emissive material defining an -accelerating path having its lonitudinal dimension substantially larger than its lateral dimension,

entrance means into the accelerating path deiined by said wall means,

exit means from said accelerating path defined by said wall means spaced from said entrance means by said longitudinal dimension,

said accelerating path being totally clear of a iield producing wire on the longitudinal axis of the accelerating path,

said accelerating path being free of any superimposed oscillating electric iield in a direction transverse to the longitudinal axis,

voltage means for producing a longitudinal electrical current ow in said wall means to establish in the accelerating path a total iield which is an electric field having a component parallel to the Wall means in its longitudinal direction with the wall means current increasing as the voltage of said voltage means increases,

whereby there occurs multiplication of incoming particles impinging on' and exciting said secondary emissive material resulting in secondary emission and in a multiplication gain vs. wall means current flow curve,

said wall means current being in the range where the gain approaches unity on said gain vs. wall means current curve andthe energy of the exciting electrons at the exit means approaches the voltage of said voltage means.

10. The apparatus ofclaim 9 with photocathode means 'for receiving a light image and emitting a corresponding electron image,

said photocathode means being placed at the entrance means end of the accelerating path to emit the electron image into the first openings of said accelerators,

phosphor means being at the exit means end of said accelerating path to receive the accelerated electrons and display alcorresponding light image.

11. The apparatus of claim 9 with enclosure means for hermetically enclosing said wall means.

No references cited.

WALTER STOLWEIN, Primary Examiner U.S. Cl. X.R. 

